Airbag systems have been developed to protect the occupant of a vehicle, in the event of a frontal collision, by rapidly inflating a cushion or a bag between the vehicle occupant and the interior of the vehicle. The inflated airbag absorbs the occupant's energy to provide a gradual, controlled ride down, and provides a cushion to distribute body loads and keep the occupant from impacting the hard vehicle interior surfaces.
The most common airbag systems presently in use include an on-board collision sensor, an inflator, and a collapsed, inflatable bag connected to the gas outlet of the inflator. The inflator typically has a metal housing which contains an electrically initiated igniter, a particulate gas generant composition, and a gas filtering system. Before it is deployed, the collapsed bag is stored behind a protective cover in the steering wheel (for a driver protection system) or in the instrument panel (for a passenger system) of a vehicle. When the sensor determines that the vehicle is involved in a collision, it sends an electrical signal to the igniter, which ignites the gas generant composition. The gas generant composition burns, generating a large volume of relatively cool gaseous combustion products in a very short time. The combustion products are contained and directed through the filtering system and into the bag by the inflator housing. The filtering system retains all solid and liquid combustion products within the inflator and cools the generated gas to a temperature tolerable to the vehicle passenger. The bag breaks out of its protective cover and inflates when filled with the filtered combustion products emerging from the gas outlet of the inflator.
The requirements of a gas generant suitable for use in an automobile airbag are very demanding. The gas generant must burn very fast to inflate the airbag in about 30 milliseconds, but the burn rate must be stable, controllable, and reproducible to insure bag deployment and inflation in a manner which does not cause injury to the vehicle occupants or damage to the bag. The burn rate of the gas generant is thus very critical.
The gas generant must be extremely reliable during the life of the vehicle (ten or more years). Ignition must be certain, and the burn rate of the gas generant composition must remain constant despite extensive exposure of the composition to vibration and a wide range of temperatures. The gas generant is protected from moisture when sealed in the inflator, but should still be relatively insensitive to moisture to minimize problems during manufacture and storage of the gas generant and assembly of the inflator, and to insure reliability during the life of the airbag system.
The gas generant must efficiently produce cool, non-toxic, non-corrosive gas which is easily filtered to remove solid or liquid particles, and thus to preclude injury to the vehicle occupants and damage to the bag.
The gas generant must have good thermal stability and long term aging characteristics to insure functionality of the airbag system over the life of the vehicle.
The requirements of the preceding paragraphs prevent many apparently suitable compositions from being used as airbag gas generants.
Mixtures of sodium azide and ferric oxide are favored from the point of view of combustion temperature, filterability of solid or liquid combustion products, volume of gas produced per weight of composition, and lack of toxic gaseous products. They have a combustion temperature of no more than 1000.degree. C., provide an efficient conversion to gas, produce almost pure nitrogen, and produce solid secondary combustion products in the form of clinkers which are easily trapped by the filtering system of the inflator.
Sodium azide and ferric oxide based gas generants have previously been less preferred than other compositions, however, because they burn unstably and slowly and are difficult to compact into tablets (U.S. Patent No. 4,203,787, issued to Kirchoff, et al. on May 20, 1980, column 2, lines 25 and following). An additional problem which has historically hindered the acceptance and usefulness of sodium azide and iron oxide based gas generants has been their propensity to absorb moisture under normal atmospheric conditions, which degrades the gas generant's physical properties and reduces its burn rate.
The compaction problem has been solved by adding molybdenum disulfide (Kirchoff, cited above, Example 6; U.S. Pat. No. 4,547,235, issued to Schneiter, et al. on Oct. 15, 1985, especially Column 3, lines 4-8).
In compositions containing no ferric oxide, the problem of unstable or slow burning has been addressed by adding a mixture of potassium nitrate and finely divided silica to the gas generant (Schneiter, et al., cited above, especially from column 2, line 50 to column 3, line 4, and in column 3, lines 8-24). The Schneiter reference has also proposed the use of sulfur to increase the burn rate, and in Example 8 provides a comparative example in which sodium azide, ferric oxide, and sulfur are combined. But the combustion products of this combination are shown by Schneiter, et al. to be more caustic than its preferred compositions, which employ a mixture of silica and potassium nitrate as a burn rate modifier. Sulfur also generates an unpleasant smell and toxic sulfur dioxide when burned.
The problem of increasing the burn rate of sodium azide and iron oxide gas generants has been addressed in many ways, such as using chemical additives and unique or special processing methods. We know of no prior art in which sodium nitrate is used as a burn rate modifier for a sodium azide and iron oxide gas generant.
U.S. Pat. No. 3,947,300, issued to Passauer, et al. on Mar. 30, 1976, teaches a gas generant comprising sodium azide, potassium nitrate, and silica. The reference suggests that lead oxide or ferric oxide can be added as "glass flux promoting oxides" (column 3, lines 7-13), but teaches away from adding large amounts of such oxides (column 3, lines 50-54).
The problem of degradation of airbag gas generants under humid conditions has not been substantially addressed in the relevant art. U.S. Pat. No. 3,996,079, issued to DiValentin on December 7, 1976, includes a perfunctory humidity test in Example 4, but does not test a composition containing ferric oxide or sodium nitrate and does not explain how the slower burning rate caused by humidity can be remedied. The humidity test in Example 4 of DiValentin was run at 0.02% by weight water vapor (1.5% relative humidity) and at 0.62% by weight water vapor (46% relative humidity). The latter value is actually lower than the average relative humidity in July in the most highly populated areas of the United States, according to the Encyclopedia Britannica, 15th Ed., Volume 9, page 3 (1980).
A trade brochure of unknown publication date, published at least by Feb. 26, 1986 by Tulco, Inc., describes its TULLANOX 500 brand of hydrophobic fumed silica. It suggests the incorporation of this material in various products, for example match heads, match striker strips, and blasting powder, to retard moisture and improve water resistance. No suggestion is made that hydrophobic fumed silica would be useful in gas generant compositions.
To summarize, no prior art has shown how airbag gas generant compositions predominantly comprising sodium azide and ferric oxide can be made to burn faster, particularly when high humidity is a factor, without forming caustic products. The advantages of sodium azide and ferric oxide based airbag gas generants thus have not previously been realized. Sodium nitrate and hydrophobic fumed silica have not previously been used or suggested for use in airbag gas generant compositions.